There is a continuing need to improve the performance of computers to meet the needs of new and more sophisticated computing applications. Applications such as pattern classification, pattern association, associative memory, speech, and character recognition remain largely unamenable to solution or implementation by current computers. Many simple tasks performed readily and intuitively by humans and other biological organisms are also beyond the capabilities of conventional computers.
The desire to expand the frontiers of computer science has motivated a consideration of the factors that contribute to the limitations of current computers. Silicon is at the heart of today's computer. The advances in computing power and speed over the years have largely been a consequence of better understanding the fundamental properties of silicon and harnessing those properties for practical effect. Initial progress was predicated on building basic electronic components such as transistors and diodes out of silicon, and later progress followed from the development of integrated circuits. More recent advances represent a continuation of these trends and currently emphasize miniaturization and integration of an ever-larger number of microelectronic devices on a single chip. Smaller devices lead to higher memory storage densities, more highly integrated circuits and faster computers.
Since future improvements in computing power and functionality are currently predicated on further improvements in silicon technology, there has been much recent discussion about the prognosis for continued miniaturization of silicon-based electronic devices. A growing consensus is emerging that believes that the computer industry is rapidly approaching the performance limits of silicon. The feature size in today's manufacturing technologies is 0.13 micron and it is expected that this can be reduced to about 0.08 micron in the future. Further decreases in feature size, however, are deemed problematic because smaller dimensions lead to a change in the fundamental behavior of silicon. More specifically, as the dimensions of silicon devices decrease to tens of nanometers and below, silicon enters the quantum regime of behavior and no longer functions according to the classical physics that governs macroscopic objects. In the quantum regime, energy states are quantized rather than continuous and phenomena such as tunneling lead to delocalization of electrons across many devices. Consequences of tunneling include leakage of current as electrons escape from one device to neighboring devices and a loss of independence of devices as the state of one device influences the state of neighboring devices. In addition to fundamental changes in the behavior of silicon, further decreases in the dimensions of silicon devices also pose formidable technological challenges. New and costly innovations in fabrication methods such as lithography will be needed to achieve smaller feature sizes.
One strategy for advancing the capabilities of computers is to identify materials other than silicon that can be used as the active medium in data processing, logic or memory applications. Such alternative computing media could be used independent of or in combination with silicon to form the basis of a new computing paradigm that seeks to offer better performance and more convenient manufacturing than is possible with silicon.
The instant inventor has recently proposed the use of chalcogenide phase change materials as an active material for the processing and storage of data. In U.S. Pat. No. 6,671,710 (the '710 patent), the disclosure of which is hereby incorporated by reference herein, Ovshinsky et al. describe a principle of operation of phase change materials in computing applications. Phase change materials not only operate in the binary mode characteristic of conventional silicon computers, but also offer opportunities for the non-binary storage and processing of data. Non-binary storage provides for high information storage densities, while non-binary processing provides for massively parallel operation. The '710 patent also describes algorithms that utilize a non-binary computing medium for mathematical operations such as addition, subtraction, multiplication and division. U.S. Pat. Nos. 6,714,954; 6,963,893; and 7,440,990 by Ovshinsky et al., the disclosures of which are hereby incorporated by reference, describe further mathematical operations based on a phase change computing medium. These operations including factoring, modular arithmetic and parallel operation.
In U.S. Pat. No. 6,999,953, the disclosure of which is hereby incorporated by reference, Ovshinsky considers the architecture of computing systems based on devices utilizing chalcogenide materials as the active computing medium or to regulate signal transmission. More specifically, Ovshinsky considers networks of chalcogenide computing and switching devices and demonstrates functionality that closely parallels that of biological neural networks. Important features of this functionality include the accumulative response of phase change computing devices to input signals from a variety of sources, an ability to weight the input signals and a stable, reproducible material transformation that mimics the firing of a biological neuron. This functionality enables a new concept in intelligent computing that features learning, adaptability, and plasticity.
In U.S. Pat. Nos. 6,967,344; (the '344 patent); 6,969,867; (the '867 patent); 7,227,170 (the '170 patent); 7,227,221 ('221 patent), and 7,186,998 ('998 patent), the disclosures of which are hereby incorporated by reference herein, Ovshinsky et al. further develop the notion of chalcogenide computing by discussing additional computing, storage, and processing devices. The '344 patent discusses a multi-terminal chalcogenide switching device where a control signal provided at one electrical terminal modulates the current, threshold voltage or signal transmitted between other electrical terminals through the injection of charge carriers. The '867 patent describes a related multi-terminal switching device that utilizes a field effect terminal to modulate the current, threshold voltage or signal transmitted between other terminals. The devices described in the '344 and '867 patents may be configured to provide functionality analogous to that of the transistor that is so vital to silicon-based computers. The '170, '221, and '998 patents present multiple-bit storage and logic devices that utilize a chalcogenide phase-change material in a multi-terminal device design.
In U.S. Pat. No. 7,085,155, Ovshinsky et al. demonstrated a device for securing data in a chalcogenide computing system. The security device exploits the variable resistivity of the phase-change material and provides a parallel circuit combination that regulates the resistance of surrounding elements in different ways for authorized and unauthorized reads of the phase-change device. The circuit combination may include a parallel combination of a chalcogenide phase-change material and a chalcogenide switching material. U.S. Pat. No. 7,186,999 by Ovshinsky et al. provides an error reduction circuit for phase-change computing based on series-parallel circuit combinations of chalcogenide devices arranged in an array, where a chalcogenide switching device may be used as an access device.
The foregoing work by Ovshinsky et al. provides a concept, operating principles and basic devices that enable a computing paradigm based in whole or in part on chalcogenide materials. In order to further the realization of chalcogenide computing as a viable complement to or alternative to silicon-based technologies, it is desirable to expand the range of devices and functionality available from chalcogenide phase change materials. Of greatest interest are devices capable of performing operations necessary for the processing and storage of data as well as systems that regulate signal transmission and access to devices. Of particular interest in enabling higher speed operation is an extension of chalcogenide computing beyond purely electrical operation to hybrid electrical-optical or pure optical operation.